Scientists led by a University of Texas geologist report that data from an unmanned NASA space probe suggests there's much more ice on Mars than previously thought.

The Mars Reconnaissance Orbiter, according to an article in the journal Science, has identified several dirt-covered glaciers — including one that is three times longer than the city of Los Angeles and up to a half-mile thick. The glaciers may be remnants of warmer conditions on the Red Planet.

Earlier this year, another spacecraft, NASA's Phoenix Mars Lander, scratched below the dirt-covered plains of the planet's North Pole to uncover a glistening slab of ice.

Mars is a cold, dry realm. But long channels and what appear to be watersheds in the rugged terrain suggest to some scientists that water once flowed or pooled on the planet.

In addition, Mars may undergo periodic climate swings linked to changes in its axis. During these changes, the north pole dips, exposing the region to higher temperatures, possibly thawing the ice.

The Mars Orbiter used radar to examine the Hellas Basin, an ancient asteroid impact region in the planet's southern hemisphere. The scientists supervising the $700 million project were interested in the area's gently sloping features at the edges of mountains and cliffs.

The radar signals from the orbiter penetrated the dirt-covered features and were reflected back at velocities consistent with radio waves passing through ice, the scientists said.

The spacecraft also spotted similar sloping formations extending from cliffs in the northern hemisphere.

If Mars once hosted a more substantial atmosphere and warmer climes, then water may have fostered some form of life. Water would prove valuable to future human explorers as well. Its chemical elements, oxygen and hydrogen, can be used as rocket propellants, and oxygen, of course, is a source of air for breathing.

"Altogether, these features almost certainly represent the largest reservoir of water ice on Mars that's not in the polar caps," Holt said.

Ali Safaeinili, a member of the research team from NASA's Jet Propulsion Laboratory, agreed. "These results are the smoking gun pointing to the presence of large amounts of water ice at these latitudes," he said.